1. Field of the Invention
The present invention relates to a dewatering apparatus for a gas hydrate slurry, and more specifically, to a dewatering apparatus in a production plant of gas hydrate in which a gas hydrate slurry is generated by being subjected to a hydration reaction of raw material gas such as methane or the like, and raw material water.
2. Description of Related Art
In recent years, natural gas which contains methane or the like as a major component has captured much of the spotlight as a clean energy source. Then, for purpose of transportation and storage, a practice of transforming such a natural gas into a liquified natural gas (hereinafter, referred to as LNG) is being conducted. Since, however, the transportation and storage of a gas in the form of a LNG requires maintaining it in a cryogenic state, not only a generation system but also a transportation system and a storage system have become quite expensive. As a consequence, they are limited to only large-scale gas fields, and were economically unfeasible for smaller-scale gas fields.
Under these circumstance, studies on manufacturing natural gas hydrate (hereinafter, simply referred to as gas hydrate) by causing natural gas to react with water, and transporting or storing it through the gas hydrate are being carried out. With regard to this gas hydrate, it is well known that the raw material gas and the raw material water are introduced into a reactor in which a predetermined temperature and pressure selected from among, for example, temperatures of 1 to 10° C. and atmospheric pressures of 30 to 100 atmosphere are retained, to generate a slurry which contains a crystalline-like gas hydrate. Then, this slurry is introduced into a dewatering apparatus to separate and remove unreacted water, and is subsequently again brought into contact with the raw material gas to manufacture a powdery gas hydrate having low water content.
In a production plant for such a gas hydrate, a horizontal screw press-type dewatering apparatus and a vertical gravity-type dewatering apparatus are proposed as a dewatering apparatus (e.g., Patent Document 1).
A horizontal screw press-type dewatering apparatus as described in such a Patent Document 1 is made of a double construction combined with a mesh-processed inner wall, and a cylindrical body constituting an outer shell situated at the outside of the inner wall, and it is configured such that a gas hydrate is drained from meshes processed on the inner wall by advancing the gas hydrate while forcedly squeezing it by a screw shaft mounted inside the inner wall.
In such a dewatering apparatus, the gas hydrate was consolidated and was adhered to the surface of a screw, during said process of dewatering said gas hydrate. As a result a load of the screw shaft was increased, and thus such a dewatering apparatus was required to be driven at a high torque.
Thus, in order to solve the problem with said dewatering apparatus, the present inventors have studied a dewatering apparatus in which the gas hydrate slurry is supplied into the cylindrical body by a slurry pump, and water is drained naturally from a porous portion of the cylindrical body while causing it to move up in succession, through the use of a vertical-type dewatering apparatus having a separating section formed to be porous at an intermediate section of a cylindrical body (e.g., Patent Documents 2, 3).
The vertical-type dewatering apparatus as described in Patent Document 2, the present inventors previously proposed, includes a cylindrical main body with drain holes formed at substantially intermediate section, and a dewatering collecting section (drainage chamber) provided around said drain holes. Then, the gas hydrate slurry supplied to the dewatering apparatus is designed to be dewatered resulting from unreacted water being drained from said drain holes.
Further a vertical-type dewatering apparatus as described in Patent Document 3, the present inventors previously proposed, is configured such that a dewatering column is made of a double cylindrical construction consisting of two cylindrical bodies of an internal tube and an external tube, and dewatering filtration elements are provided on both side walls of the internal tube and external tube respectively, then the unreacted water is caused to outflow to the outside of the column through both the filtration elements provided on the internal tube and the external tube.
Incidentally, since a dewatering apparatus as described in said Patent Document 2 is configured such that water and hydrate are separated by the action of gravity, there was a problem of slow rates at which the unreacted water is drained from said drain holes. In addition, the dewatering column must be high enough to enhance dewatering efficiency, and thus there was a problem with the increase in size of the apparatus.
A dewatering column as described in the other Patent Document 3 includes an annular-shaped bottom plate, an annular-shaped shielding plate, a gas hydrate-crushing device, and plural tabular blades provided in radial form at the lower end and so on, to form a complicated construction. Therefore, there was a problem that a period required to manufacture the dewatering column becomes longer, along with a higher cost.    Patent Document 1: Japanese Patent Application Kokai Publication No. 2003-105362    Patent Document 2: Japanese Patent Application Kokai Publication No. 2006-111769    Patent Document 3: Japanese Patent Application Kokai Publication No. 2006-257359